Hapten 

Hapten, also known as incomplete antigen, refers to simple small molecule antigen that only has immune response specificity but no immunogenicity, such as certain polysaccharides, lipids, and drugs. Haptens are small molecules that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself. Attaching of haptens to a carrier molecule lead to a complete antigen. Haptens have been used to study allergic contact dermatitis (ACD) and the mechanisms of inflammatory bowel disease (IBD) to induce autoimmune-like responses.